docs: def covariance(self, param_1, param_2):
          """
          Return the covariance between param_1 and param_2.
  
          :param param_1: ``Parameter`` Instance.
          :param param_2: ``Parameter`` Instance.
          :return: Covariance of the two params.
          """
          param_1_number = self.model.params.index(param_1)
          param_2_number = self.model.params.index(param_2)
          return self.covariance_matrix[param_1_number, param_2_number]